tumor immunology

Recently someone asked about the concept of immunosurveillance of cancer,
to which I replied that I didn't think the immune system could survey the
body for the development of malignancies because tumors arise from
tissues that cannot deliver costimulatory signals for the activation of
naive T cells.
This response provoked two questions:
1) What about "cross-priming"?
2) What if such tissues were infected by viruses?
Sorry I didn't answer earlier. I have been having trouble posting.
Here are my answers:
1) As small tumors grow, they tend not to shed tumor-specific antigens.
If they do, they do so in small amounts, and the cells that are best
suited to picking up low concentrations of antigen are antigen-specific B
cells, which are also tolerogenic APCs for naive T cells. By the time a
tumor has outgrown its blood supply, it has already tolerized all tumor
antigen-specific T cells and so there is no cross-priming.
2) I believe that a virus that infects a non-professional APC and does
not cause any tissue destruction will either go undetected by the immune
system or render it tolerant. Only those viruses that cause destruction
will allow "professional" APCs to pick up viral antigens to incite an
immune response. This is a good mechanism by which the body can pick up
useful genes without the immune system trying to eliminate them.